Recently the superconducting semimetal FeSe [1]}, [2]}, [3]} with very small hole and electron Fermi surfaces is found to exhibit large ratios of the superconducting energy gap \(\Delta \)  and Fermi energy \(\varepsilon _{\rm F}\) , \(\Delta /\varepsilon _{\rm F} \approx 0.3\) -1.0 [4]}, [5]}, [6]}, which appears to place FeSe in the crossover regime (Fig.REF (a)). Indeed, in high-quality single crystals with long mean free paths of electrons [4]}, large superconducting fluctuations have been reported from the torque magnetometry, providing support for the presence of fluctuated Cooper pairs created above \(T_{\rm c}\)  in the crossover regime [8]}. These observations underline the importance of the BCS-BEC crossover physics both in the superconducting and normal states of FeSe.
On the other hand, scanning tunneling spectroscopy (STS) measurements do not observe the pseudogap formation above \(T_{\rm c}\)  [9]}.
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